64 Mr. hunter on a new Method 
Thus, for example, fuppofc I am to meafure the diameter of 
a nervous fibre in the medullary fubftance of the brain, I make 
the point k appear clofe to one edge, and turn the index till the 
fame point pafs over the fibre, and appear to touch the other 
edge : I then look on the graduated circle ecd, and perceive 
that the index c has palled over, fuppofe, 23,2 divifions. Hence 
I conclude the diameter of the fibre to be 23,2 y - _ = _ ' 
of an inch, which is nearly the fize as found by the accurate 
obfervations of Dr. monro. There fhould be a nonius’s fcale 
on the index which will meafure to one tenth of a divifion. 
As the index c mull continue clofe to the plate ecd, while at 
the fame time it turns round the fcrew ab, which is continually 
lfifing, it muft be made as in fig. 4. where a , b , are two fmall 
pieces which play in a groove in the fcrew ab (fig. 3.) while the 
groove cd (fig. 4.) in the index is filled up by a protuberance of 
the plate ecd (fig. 3 ) ; the piece below the groove cd (fig. 4.) 
being funk into that plate. The whole machinery may be in- 
clofed in a cylinder of brafs reaching from b to l (fig. 3.), fo 
that the point of the fcrew kl maybe without it, and the fides 
of the cylinder may be open at ecd. 
It is farther to be obferved,that what has been faid goes on the 
fuppofition that the point k, in the micrometer, is equally mag- 
nified with the objedtwe are to meafure. But, if this point be 
placed in the focus of the eye-glafs of a double microfcope ; 
when it moves it will pafs over, not the objedl itfelf, but its 
image, magnified by the objedl-glafs. In this cafe, if thp objedt- 
glafs magnify the diameter 1 o times, while the index pafles 
Dver one divifion, the point k will pafs over the image of an 
objedt, the diameter of which is of an inch. As in 
this mode of application the point k muft fall between the 
objedl 
